Cellular radio system allowing mobile station to perform communication through base station to which mobile station is connected over cdma radio channel, and base station apparatus and mobile station apparatus which are used for cellular radio system

ABSTRACT

A CPU ( 42   a ) in each base station (BS) monitors the channel occupancy of each of a plurality of radio frequencies allocated to the base station (BS). 
     When the difference between the channel occupancies of a plurality of radio frequencies allocated to a single base station (BS) becomes a predetermined state, the CPU ( 42   a ) switches the radio frequency as a candidate to be used by a predetermined base station (BS) of mobile stations (BS) that are present in the cell formed by the base station (BS) and set in the standby state to a radio frequency whose channel occupancy is in a predetermined state.

TECHNICAL FIELD

The present invention relates to a cellular radio system such as acar/portable telephone system and a cordless telephone system and, moreparticularly, to a cellular radio system using a code division multipleaccess (CDMA) scheme as a radio access scheme between a base station anda mobile station, and a base station apparatus and mobile stationapparatus which are used for the cellular radio system.

BACKGROUND ART

A spread spectrum communication scheme, which is robust againstinterference and disturbance, has gained a great deal of attention asone of communication schemes used for a mobile communication system. Thespread spectrum communication scheme is mainly used to implement acellular radio system using CDMA scheme.

In the cellular radio system using CDMA scheme, for example, thetransmitting apparatus modulates digital speech data or digital imagedata by a digital modulation scheme such as PSK. This modulatedtransmission data is then converted into a broadband baseband signal byusing a spreading code such as a pseudorandom noise code (PN code). Thisbaseband signal is up-converted into a signal in the radio frequencyband and is then transmitted. The receiving apparatus down-converts thereceived signal in the radio frequency band into a signal having anintermediate or baseband frequency. The apparatus then de-spreads thisdown-converted signal by using the same spreading code as that used bythe transmitting apparatus. Digital demodulation is then performed forthe de-spread signal by a digital demodulation scheme such as PSK,thereby reconstructing data from the received data.

That is, in CDMA scheme, different spreading codes are assigned to radiocommunications between a plurality of mobile station apparatuses andbase stations to ensure channel separation between the respective radiocommunications.

FIG. 14 shows the schematic arrangement of a CDMA cellular radio system.Referring to FIG. 14, a plurality of base stations BS1 to BSn aredistributed in a service area. These base stations BS1 to BSn arerespectively connected to a control station CS through wire lines L1 toLn. The base stations BS1 to BSn are further connected to a wire networkNW through the control station CS. The base stations BS1 to BSnrespectively form radio zones Z1 to Zn called cells. Mobile stations MS1to MSm are respectively connected to base stations BS in the cells wherethe respective mobile stations are present by CDMA scheme over radiopaths.

In a system of this type, when any one of the mobile stations MS1 to MSmmoves between cells during communication, handoff, i.e., the switchingof the base station to another to which radio path is to be connected isperformed. There are two types of handoff: soft handoff and hardhandoff.

Soft handoff is unique to a CDMA cellular radio system. The performhandoff, a mobile station has two radio paths at the same time. One ofthe radio paths connects the mobile station to the source base stationwith which it has been communicating. The other radio path connects themobile station to the destination base station with which it willcommunicate. The mobile station then performs path diversity synthesis,by using the signals it receives over these radio paths. Thereafter, ofthe paths under path diversity synthesis, a path on which the receptionelectric field strength of a pilot channel has dropped below a thresholdfor a predetermined period of time or more is disconnected, therebyswitching the base stations to which the mobile station is connected. Asdescribed above, in a soft handoff, one of two paths is always connectedto a base station at the time of handoff, and hence no pathdisconnection occurs. An advantage of soft handoff is that switching canbe smoothly performed without any short break in speech.

Soft handoff, however, requires both the source and destination basestations to use the same radio frequency. For this reason, for example,as shown in FIG. 14, in a system in which different radio frequenciesf1, f2, and f3 are allocated to a plurality of base station groups BSa,BSb, and BSc, when a mobile station MSi moves from a cell of the basestation group Bsa to a cell of another base station group BSb or BSc,soft handoff cannot be performed.

In contrast to this, hard handoff is mainly performed when the abovesource and destination base stations use different frequencies. Morespecifically, when a mobile station must change the radio frequency inuse at the time of handoff, a message for instructing handoff is sentfrom a base station to the mobile station. Upon reception of thismessage, the mobile station temporarily stops transmission/reception andforms a new radio path allocated by the base station between itself andthe base station. After this radio path is formed, the mobile stationresumes transmission/reception by using the path. In a hard handoff, theradio path is temporarily disconnected to switch the radio frequencies,and a radio path must be formed again by using a new radio frequency.

In such a system, a plurality of radio frequencies may be allocated tothe respective base station groups, and the respective base stations mayuse CDMA scheme with the respective radio frequencies, therebyincreasing the number of traffic channels.

In this case, however, the occupancies of the traffic channels havingthe respective radio frequencies in the respective base stations maybecome uneven. Assume that the occupancies of the traffic channelshaving the respective radio frequencies become uneven. In this case, ifan originating mobile station selects a radio frequency with densetraffic, the base station may be found busy in spite of the presence ofan available channel, resulting in blocked communication. Assume alsothat a mobile station selects a radio frequency with dense traffic in ahard or soft handoff. In this case, even if an available channel ispresent, it takes a long period of time to form a radio path again. As aresult, speech communication may be interrupted or noise may beproduced, deteriorating the speech communication quality. Alternatively,the call may be dropped due to a handoff failure.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a cellular radiosystem which can average the traffic channel occupancies of therespective radio frequencies, thereby allowing effective use of trafficchannels.

In order to achieve the above object, according to the presentinvention, in a cellular radio system including a plurality of basestations respectively belonging to a plurality of base station groupsand forming cells having a predetermined diameter, and mobile stationsconnected to the base stations over CDMA radio channels, each of thebase stations using CDMA radio channels of a plurality of radiofrequencies allocated to the base station group to which the basestation belongs, the channel occupancies of a plurality of radiofrequencies allocated to each of the plurality of base stations aremonitored by a channel occupancy monitoring means.

When the difference between the channel occupancies of the respectiveradio frequencies allocated to the same base station, determined by thechannel occupancy monitoring means, becomes a predetermined state, theradio frequency as a candidate to be used by a predetermined mobilestation of mobile stations that are present in the cell formed by thebase station and set in the standby state is switched to the radiofrequency whose channel occupancy determined by the channel occupancymonitoring means is in a predetermined state.

Consequently, as radio frequencies to be used by mobile stations in thestandby state to newly start communications, radio frequencies whosechannel occupancies are in a predetermined state are properlydistributed at random, thereby averaging the channel occupancies of therespective radio frequencies.

In addition, according to the present invention, a frequency use statenotifying means generates frequency use state information for notifyingthe mobile station of the use state of each radio frequency on the basisof the channel occupancies determined by the channel occupancymonitoring means, and transmits the information to the mobile station.The mobile station forms a radio frequency list in which priority levelsare assigned to a plurality of radio frequencies on the basis of thefrequency use state information before power-off, and stores it in apower-off state. Immediately after power-on of the mobile station, asearch is performed to check in the order of priority levels whether theradio frequencies indicated by the radio frequency list stored in thestorage means can be used, and the first detected radio frequency thatcan be used is as a candidate radio frequency to be used.

Consequently, as radio frequencies to be used by mobile stations in thestandby state to newly start communications, radio frequencies areproperly distributed at random on the basis of the use state of eachradio frequency before power-off, thereby averaging the channeloccupancies of the respective radio frequencies.

When the mobile station moves to a new cell formed by another basestation belonging to the same base station group to which the basestation forming an old cell in which the mobile station has been presentbelongs, it is checked, on the basis of the channel occupancy determinedby the channel occupancy monitoring means with respect to the basestation forming the new cell, whether the channel occupancy of the radiofrequency in the new cell, which has been used by the mobile station inthe old cell, is not less than a predetermined value. Soft handoff isdetermined if the channel occupancy of the radio frequency in the newcell, which has been used by the mobile station in the old cell, is lessthan predetermined value. Hard handoff is determined if the channeloccupancy of the radio frequency in the new cell, which has been used bythe mobile station in the old cell, is not less than predeterminedvalue. When hard handoff is to be performed, a radio frequency whosechannel occupancy is not more than a predetermined value in the new cellis used in the mobile station.

Consequently, as radio frequencies to be used after mobile handoff,radio frequencies whose channel occupancies are not more than thepredetermined value in new cells are properly distributed at random,thereby averaging the channel occupancies of the respective radiofrequencies.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing the schematic arrangement of a CDMA cellularradio system according to an embodiment of the present invention;

FIG. 2 is a block diagram showing the arrangement of a mobile station MSused in the system in FIG. 1;

FIG. 3 is a block diagram showing the arrangement of a base station BSused in the system in FIG. 1;

FIG. 4 is a flow chart showing a control procedure for idle handoffcontrol in a CPU 42 a;

FIG. 5 is a view showing a first determination condition for determiningthe necessity of idle handoff;

FIG. 6 is a view showing a second determination condition fordetermining the necessity of idle handoff;

FIG. 7 is a view showing a third determination condition for determiningthe necessity of idle handoff;

FIG. 8 is a flow chart showing a control procedure for idle handoffcontrol in a CPU 13 a;

FIG. 9 is a flow chart showing a control procedure for control atpower-off in the CPU 13 a;

FIG. 10 is a flow chart showing a control procedure for start-up controlin the CPU 13 a;

FIG. 11 is a flow chart showing a control procedure for mobile handoffcontrol in the CPU 13 a and the CPU 42 a;

FIG. 12 is a view for explaining mobile handoff control;

FIGS. 13A and 13B are views each showing a modification of thearrangement of the base station BS; and

FIG. 14 is a view showing the schematic arrangement of a CDMA cellularradio system.

BEST MODE OF CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below withreference to the accompanying drawings.

FIG. 1 shows the schematic arrangement of a CDMA cellular radio systemaccording to this embodiment. The apparent arrangement of this system isthe same as that shown in FIG. 14. However, this system differs fromthat shown in FIG. 14 in the allocation of radio frequencies to basestation groups BSa to BSc and the handoff control function of the systemas follows.

The system of this embodiment has a total of six radio frequencies f1 tof6. The radio frequencies f1 and f2 are allocated to the base stationgroup BSa; the radio frequencies f3 and f4, to the base station groupBSb; and the radio frequencies f5 and f6, to the base station group BSc.Note that the radio frequencies f1 to f6 are respectively composed ofupstream carriers fU1 to fU6 for transmitting signals from mobilestations MS (MS1 to MSm) to base stations BS (BS1 to BSm) and downstreamcarriers fD1 to fD6 for transmitting signals from the base stations BSto the mobile stations MS. An upstream carrier fUk (k is 1 to 6) and adownstream carrier fDk satisfy:

fUk=FDk+[predetermined frequency offset]

Each base station BS transmits a pilot channel, a sync channel, a pagingchannel, and a downstream traffic channel with the above downstreamcarriers fD. The mobile station MS transmits an access channel and anupstream traffic channel with the above upstream carriers fU.

This system performs radio communication based on CDMA scheme betweenthe base station BS and the mobile station MS by selectively using radiofrequencies allocated to the base station BS. Various control operationsassociated with this radio communication include mobile handoff controlto be performed when the mobile station MS moves between the cells ofthe base stations BS, and idle handoff control to be performed when thetraffic of a specific radio frequency considerably increases as comparedwith the traffic of another radio frequency allocated to the same basestation BS.

FIG. 2 is a block diagram showing the arrangement of the mobile stationMS used in the above CDMA cellular radio system.

As shown in FIG. 2, the mobile station MS includes a microphone 10 a, aspeaker 10 b, an analog-digital converter (to be referred to as an A-Dconverter hereinafter) 11 a, a digital-analog converter (to be referredto as a D-A converter hereinafter) 11 b, a voice coder-decoder (to bereferred to as a vocoder hereinafter) 12, a mobile station controlsection 13, a data generating circuit 14, a convolutional encoder 15, aninterleave circuit 16, a spread spectrum unit 17, a digital filter 18, adigital-analog converter (to be referred to as a D-A converterhereinafter) 19, an analog front end 20, an antenna 21, ananalog-digital converter (to be referred to as an A-D converterhereafter) 22, a search receiver (to be referred to as a search receiverhereafter) 23, an automatic gain control (AGE) circuit 24, fingercircuits 25, 26, and 27, a symbol combiner 28, a de-interleave circuit29, a Viterbi decoder 30, an error correction circuit 31, akeypad/display 32, and a memory section 33.

A speaker's transmission speech signal output from the microphone 10 ais converted into a digital signal by the A-D converter 11 a and iscoded by the vocoder 12. The mobile station control section 13 adds acontrol signal and the like to the coded transmission signal output fromthe vocoder 12 to generate transmission data.

The data generating circuit 14 adds an error detection code and an errorcorrection code to this transmission data. The transmission data outputfrom the data generating circuit 14 is coded by the convolutionalencoder 15. The interleave circuit 16 performs interleave processing forthe transmission data output from the convolutional encoder 15. Thetransmission data output from the interleave circuit 16 isspectrum-spread into a broadband signal by the spectrum spreader 17using PN and Walsh codes. The digital filter 18 removes unnecessaryfrequency components from this spectrum-spread transmission data. Thetransmission data output from the digital filter is converted into ananalog transmission signal by the D-A converter 19. This analogtransmission signal is up-converted into a signal having a predeterminedradio channel frequency and power-amplified to a predeterminedtransmission power level by the analog front end 20. Thereafter, thesignal is transmitted from the antenna 21 to the base station BS.

On the other hand, a radio signal received by the antenna 21 islow-noise-amplified and down-converted into a signal having anintermediate frequency or baseband frequency by the analog front end 20.The received signal output from the analog front end 20 is convertedinto a digital signal at a predetermined sampling period by the A/Dconverter 22. The received transmission data output from the A-Dconverter 22 is then input to the search receiver 23, the automatic gaincontrol circuit 24, and the three finger circuits 25, 26, and 27.

Each of the finger circuits 25, 26, and 27 includes an initial capturingsection, a clock tracking section, and a data demodulation section. Thedata demodulation section de-spreads the spectrum of the receivedtransmission signal from the base station BS, and integrates theresultant data through the integrating dump filter for a one-symbolperiod. Note that the three finger circuits are used to receive amultipath reception signal at a high S/N ratio by using the pathdiversity effect, and to switch base stations BS, to which the mobilestation is connected, during communication without disconnecting theradio path, i.e., to perform a so-called soft handoff.

The respective symbols demodulated by the finger circuits 25, 26, and 27are input to the symbol combiner 28, together with synchronizationinformation, to be synthesized. The synthesized demodulated symbol isinput to the de-interleave circuit 29, together with timing information,to be subjected to de-interleave processing in the de-interleave circuit29. The demodulated symbol after this de-interleave processing isViterbi-decoded by the Viterbi decoder 30. The demodulated symbol afterthis Viterbi-decoding is subjected to error correction decodingprocessing in the error correction circuit 31 to become received data.The received data is input to the mobile station control section 13. Themobile station control section 13 separates the input received data intospeech data and control data. The speech data is speech-decoded by thevocoder 12 and converted into an analog signal by the D/A converter 11b. The analog signal is then output as speech from the speaker 10 b.

The keypad/display 32 is used by the user to input dial data, controldata, and the like, and serves to display various information associatedwith the operation state of the mobile station MS. The operation of thekeypad/display 32 is controlled by the mobile station control section13.

The memory section 33 is used to store various data required when themobile station control section 13 performs various operations, and has anonvolatile storage medium such as an EEPROM. This memory section 33stores a radio frequency list (to be described later) in a storage areaformed from the nonvolatile storage medium.

The search receiver 23 basically has the same arrangement as that ofeach of the finger circuits 25, 26, and 27. The search receiver 23searches the PN codes of pilot signals broadcasted from the basestations BS in units of radio frequencies to capture the offsets of thePN codes. The power control data obtained by this PN code searchingoperation is loaded into the mobile station control section 13.

The mobile station control section 13 has a CPU 13 a, a ROM 13 b, a RAM13 c, and an I/O port 13 d connected to each other through a system bus13 e.

The CPU 13 a operates on the basis of the programs stored in the ROM 13b, and collectively controls the respective sections of this mobilestation MS, thereby implementing the operation of the mobile station MS.

The ROM 13 b stores the operation programs and the like for the CPU 13a.

The RAM 13 c temporarily stores data required for the CPU 13 a toperform various operations.

The functions implemented by the CPU 13 a by means of softwareprocessing include a candidate frequency setting function, a radiofrequency list forming function, a candidate frequency initializingfunction, and a mobile-station-side handoff processing function, as wellas the known general control function of the mobile station MS.

The candidate frequency setting function serves to set a radio frequencyto be used when communication is started next, i.e., a candidatefrequency to be used as a radio frequency in the standby state. Thiscandidate frequency setting function includes the function of changingcandidate frequencies in accordance with an idle handoff instructionfrom the base station BS, i.e., the function of performing idle handoff.

The radio frequency list forming function serves to receive frequencyuse state information from the nearest base station BS, which indicatesthe use state of radio frequencies in the cell formed by the basestation BS and the use state of radio frequencies in a predeterminedneighboring cell, and form a radio frequency list indicating thepriority levels of the respective radio frequencies on the basis of thefrequency use state information. The radio frequency list formingfunction serves to store this formed radio frequency list in the memorysection 33.

The candidate frequency initializing function serves to initializecandidate frequencies by referring to the radio frequency list stored inthe memory section 33 when the power to the mobile station MS is turnedon.

When the mobile station MS moves from the cell of a given base stationBS to the cell of another neighboring base station BS, themobile-station-side handoff control function serves to perform control,together with these base stations BS, to switch a first radio pathconnecting the mobile station MS to the source base station BS to asecond radio path connecting the mobile station MS to the destinationbase station BS. At this time, the mobile-station-side handoff controlfunction performs soft handoff control when the radio frequency of thefirst radio path is equal to that of the second radio path. Assume thatin hard handoff control, allocated radio frequency notificationinformation for notifying the radio frequency to be used to connect thesecond radio path is supplied from the source base station BS. In thiscase, the mobile-station-side handoff control function serves to connectthe second radio path by using this radio frequency.

FIG. 3 is a block diagram showing the arrangement of the base stationBS.

As shown in FIG. 3, the base station BS includes a control stationinterface section 41, a base station control section 42, p (p is thenumber of traffic channels for one radio frequency) CDMA modulationsections 43 (43-1 to 43-p) for the first radio frequency, p CDMAmodulation sections 44 (44-1 to 44-p) for the second radio frequency, apilot signal generating section 45, synthesizers 46 and 47, an analogfront end 48, an antenna 49, p CDMA demodulation sections 50 (50-1 to50-p) for the first radio frequency, and p CDMA demodulation sections 51(51-1 to 51-p) for the second radio frequency.

The control station interface section 41 transmits/receives speech dataand control data to/from the control station CS. For example, speechdata sent in a time-divisionally multiplexed state from the controlstation CS is demultiplexed by the control station interface section 41.Each of the demultiplexed speech data is converted into data in a dataform for transmission on a radio path by the control station interfacesection 41.

The speech data having undergone data conversion are parallelly suppliedto the base station control section 42. The base station control section42 adds control signals and the like to the respective speech data togenerate transmission data.

These transmission data are input to CDMA modulation sections 34 and 44for the corresponding traffic channels. Each of CDMA modulation sections34 and 44 has circuits similar to the data generating circuit 14, theconvolutional encoder 15, the interleave circuit 16, the spread spectrumunit 17, the digital filter 18, and the D-A converter 19 in the mobilestation MS. The transmission data are therefore subjected to addition oferror detection codes and error correction codes, convolutional coding,interleave processing, spread spectrum processing, and conversion toanalog signals in CDMA modulation sections 43 and 44. As a result,analog transmission signals are obtained. Note that CDMA modulationsections 43-1 to 43-p use different Walsh codes corresponding to therespective traffic channels in spreading the signal spectrum. Inaddition, CDMA modulation sections 44-1 to 44-p use different Walshcodes corresponding to the respective traffic channels in spreading thesignal spectrum.

The analog transmission signals obtained by the respective CDMAmodulation sections 43 are synthesized with each other by thesynthesizer 46. At this time, the synthesizer 46 also synthesizes apilot channel signal generated by the pilot signal generating section 45with the above signals. The analog transmission signals obtained by therespective CDMA modulation sections 44 are synthesized with each otherby the synthesizer 47. At this time, the synthesizer 47 also synthesizesa pilot channel signal generated by the pilot signal generating section45 with the above signals. Each pilot channel signal contains a PN code(common to CDMA modulation sections 43 and 44) used by CDMA modulationsections 43 and 44 for spread spectrum processing.

Both the output signals from the synthesizers 46 and 47 are input to theanalog front end 48. The output signal from the synthesizer 46 isup-converted into the first radio frequency and power-amplified to apredetermined transmission power level by the analog front end 48. Theoutput signal from the synthesizer 47 is up-converted into the secondradio frequency and power-amplified to a predetermined transmissionpower level by the analog front end 48. The output signals from theanalog front end 48 are transmitted from the antenna 49 to the mobilestation MS by radio. Note that the first radio frequency is one of thetwo radio frequencies allocated to the base station, and the secondradio frequency is the other of the two radio frequencies allocated tothe base station. That is, for example, in the base station BS belongingto the base station group BSa, the first radio frequency is f1, and thesecond radio frequency is f2.

An RF signal obtained when a radio signal is received through theantenna 49 is supplied to the analog front end 48. This RF signal islow-noise-amplified by the analog front end 48. Signals in the bands ofthe two radio frequencies allocated to the base station BS are extractedfrom this high-frequency signal. These extracted signals aredown-converted into intermediate frequencies or baseband frequencies bythe analog front end 48. The resultant signals are reception signals.

Of these reception signals, the signal extracted from the first radiofrequency band is branched/input to CDMA demodulation sections 50-1 to50-p. Of the reception signals, the signal extracted from the secondradio frequency band is branched/input to CDMA demodulation sections51-1 to 51-p.

Each of CDMA demodulation sections 50 and 51 includes circuits similarto the A-D converter 22, the search receiver 23, the automatic gaincontrol circuit 24, the finger circuits 25, 26, and 27, the symbolcombiner 28, the de-interleave circuit 29, the Viterbi decoder 30, andthe error correction circuit 31 in the mobile station MS. Receptionsignals are therefore subjected to conversion to digital signals, spreadspectrum processing, integration for a one-symbol period, symbolsynthesis, de-interleave processing, Viterbi decoding, and errorcorrection decoding processing in CDMA demodulation sections 50 and 51.As a result, reception data are obtained and parallelly input to thebase station control section 42. In this case, CDMA demodulationsections 50-1 to 50-p use different Walsh codes corresponding to therespective traffic channels in spreading the signal spectrum. CDMAdemodulation sections 51-1 to 51-p use different Walsh codescorresponding to the respective traffic channels in de-spreading thesignal spectrum. As a result, in CDMA demodulation sections 50 and 51,reception data received through the corresponding to traffic channelsare extracted.

The base station control section 42 separates the respective receptiondata into speech data and control data. Of these data, the speech dataare converted into data in a data form for transmission through atransmission path between itself and the control station by the controlstation interface section 41. The respective speech data havingundergone data conversion are transmitted in, for example, atime-divisional multiplexed state to the control station CS.

The base station control section 42 has a CPU 42 a, a ROM 42 b, a RAM 42c, and an I/O port 42 d connected to each other through a system bus 42e.

The CPU 42 a operates on the basis of the programs stored in the ROM 42b, and collectively controls the respective sections of this basestation BS, thereby implementing the operation of the base station BS.

The ROM 42 b stores operation programs and the like for the CPU 42 a.

The RAM 42 c temporarily stores data required for the CPU 42 a toperform various operations.

Note that the functions implemented by the CPU 42 a by means of softwareprocessing include a channel occupancy monitoring function, an idlehandoff controlling function, a frequency use state notifying function,a handoff method determining function, and a network-side handoffcontrol function, as well as a known general control function in thebase station BS.

In this case, the channel occupancy monitoring function serves tomonitor the channel occupancies of radio frequencies allocated to thebase station BS.

When the difference between the channel occupancies of the respectiveradio frequencies allocated to the base station BS becomes apredetermined state, the idle handoff controlling function serves tooutput an idle handoff instruction to the mobile station MS in thestandby state in the cell formed by the base station BS to set the radiofrequency with the lower channel occupancy as a candidate frequency.

The frequency use station notifying function serves to generatefrequency use state information indicating the use state of the radiofrequencies in the cell of the base station BS and the use state of theradio frequencies in a predetermined neighboring cell, and performprocessing for transmission of the information to the mobile station MS.

The handoff method determining function serves to determine whether toperform soft handoff or hard handoff for the mobile station MS, when themobile station moves from the cell of the base station BS to the cell ofanother base station BS belonging to the same base station group.

The network-side handoff control function serves to perform handoffcontrol to perform handoff for the mobile station MS by the methoddetermined by the handoff method determining function when the mobilestation MS moves from the cell of the base station BS to the cell ofanother base station BS belonging to the same base station group. Thenetwork-side handoff control function includes the function of notifyingthe mobile station MS of the radio frequency with a channel occupancylower than that of the radio frequency that has been used by the mobilestation MS in the cell it is entering, when the method determined by thehandoff method determining function is hard handoff.

The operation of the system having the above arrangement will bedescribed next.

Idle handoff control will be described first.

FIG. 4 is a flow chart showing a control procedure for idle handoffcontrol in the CPU 42 a.

The CPU 42 a in each base station BS performs idle handoff control at apredetermined timing, e.g., at predetermined periods. In this idlehandoff control, first of all, the CPU 42 a monitors the occupancies(channel occupancies) of the downstream traffic channels of therespective radio frequencies allocated to the self-station duringoperation (step ST1). The base station control section 42 checks, on thebasis of the monitoring result on the channel occupancies, whether thecurrent state corresponds to any one of the following three conditions(steps ST2 to ST5).

(1) For example, as shown in FIG. 5,

the difference between the channel use ratios of the two radiofrequencies exceeds 20%, and

neither of the channel occupancies of the two radio frequencies exceeds95%.

(2) For example, as shown in FIG. 6,

one of the channel occupancies of the two radio frequencies exceeds 95%,

the other of the channel occupancies of the two radio frequencies isequal to or lower than 94%, and

the difference between the channel use ratios of the two radiofrequencies does not exceed 20%.

(3) For example, as shown in FIG. 7,

one of the channel occupancies of the two radio frequencies exceeds 95%,

the other of the channel occupancies of the two radio frequencies isequal to or lower than 94%, and

the difference between the channel use ratios of the two radiofrequencies exceeds 20%.

These three conditions are conditions for determining whether idlehandoff is necessary. When the current state corresponds to any one ofthem, it indicates that idle handoff is necessary.

If the CPU 42 a determines that idle handoff is necessary, and thedetermination is based on condition (1) or (2), the number of idlemobile stations is set to “1” (step ST6). If the determination is basedon condition (3), the number of idle mobile stations is set to “2” (stepST7).

Subsequently, the CPU 42 a checks whether the count value of an idlehandoff counter for counting the number of times idle handoff isperformed corresponds to any one of the following three conditions(steps ST8 and ST9):

(1) smaller than “20”,

(2) equal to or larger than “2” and smaller than “25”, and

(3) equal to or larger than “25”.

If the count value of the idle handoff counter corresponds to condition(2), the CPU 42 a changes the number of idle mobile stations to a numbertwice the number set in step ST6 or ST7 (step ST10). If the countervalue of the idle handoff counter corresponds to condition (3), the CPU42 a changes the number of idle mobile stations to a number four timesthe number set in step ST6 or ST7 (step ST11). If the count value of theidle handoff counter corresponds to condition (2), the CPU 42 a keepsthe number of idle mobile stations equal to the number set in step ST6or ST7.

Subsequently, the CPU 42 a randomly selects the mobile stations MS equalin number to the number set at this time point from the mobile stationsMS that are located in the cell of the self-base station BS and in thestandby state. The CPU 42 a sends an idle handoff instructioncontrolling a radio frequency with a lower channel occupancy to each ofthe selected mobile stations MS (step ST12). When this idle handoffinstruction sending operation is complete, the CPU 42 a increments thecount value of the idle handoff counter by one (step ST13). Thereafter,idle handoff control is terminated.

If the CPU 42 a determines in steps ST2 to ST6 that none of conditions(1) to (3) is satisfied, the CPU 42 a terminates this idle handoffcontrol without sending any idle handoff instruction. If, however, thereis no radio frequency whose channel occupancy exceeds 95%, and thedifference between the two radio frequencies does not exceed 20%, theCPU 42 a terminates the processing after resetting the idle handoffcounter (step ST14).

Upon reception of the idle handoff instruction transmitted from the basestation BS in the above manner, the CPU 13 a of the mobile station MSstarts idle handoff control as shown in FIG. 8.

Upon reception of the idle handoff instruction, the CPU 13 a recognizesthe contents of the idle handoff instruction first, and then determinesthe designated radio frequency (step ST21). The CPU 13 a checks whetherthe designated radio frequency differs from the current candidatefrequency (step ST22).

If the designated radio frequency differs from the current candidatefrequency, the CPU 13 a captures the pilot channel of the designatedradio frequency (step ST23). This pilot channel capturing operation isperformed by acquiring the phase and PN timing through the searchreceiver 23. Upon completion of the pilot channel capturing operation,the CPU 13 a receives a sync channel to acquire information indicatingthe system configuration and the system timing, and also captures apaging channel (step ST24).

Upon completion of the above processing, the CPU 13 a returns to thestandby state. If the designated radio frequency is equal to the currentcandidate frequency, the CPU 13 a maintains the same standby state asthat has been set until now without performing the processing in stepsST23 and ST24. With this operation, the mobile station MS is set in thestandby state in which the radio frequency designated by the idlehandoff instruction is set as a candidate frequency.

This operation raises the possibility that a radio frequency with alower channel occupancy will be used for communication from now on,although the channel occupancies of the respective radio frequencies donot change instantly. Eventually, therefore, the channel occupancies ofthe respective radio frequencies are averaged. When the channeloccupancies of the respective radio frequencies are averaged, availabletraffic channels can be ensured in the respective radio frequencies inmany cases, thereby reliably handling new calls and mobile handoffs.

Control at power-off and start-up control at power-on in the mobilestation MS will be described next.

When the user designates power-off in the mobile station MS, the CPU 13a of the mobile station MS starts control at power-off as shown in FIG.9.

Upon reception of a power-off instruction, the CPU 13 a performsreception processing for frequency use state information first (stepST31).

Note that the CPU 42 a of each base station BS performs use statenotification control as shown in FIG. 9 at a predetermined timing, e.g.,at predetermined periods or the time required by the mobile station. Inthis use state notification control, first of all, the CPU 42 a checksthe use states of radio frequencies in the self-station and apredetermined neighboring base station (step ST41). The CPU 42 a thengenerates frequency use state information for notifying the use state ofradio frequencies in each base station, and transmits the information tothe mobile station MS (step ST42).

In the mobile station MS, the CPU 13 a outputs a request to the basestation BS or waits for frequency use state information to receive thefrequency use state information transmitted from the base station BS inthe above manner in step ST31. Upon completion of the reception of thefrequency use state information, the CPU 13 a generates a radiofrequency list in consideration of the use states of radio frequenciesin the respective base stations, indicated by the frequency use stateinformation, and stores the list in the memory section 33 (step ST32).

In this radio frequency list, priority levels are assigned to therespective radio frequencies indicated by the frequency use stateinformation in accordance with a predetermined rule. For example, thehighest priority level (“1”) is assigned to a radio frequency set as acandidate frequency, and lower priories are respectively assigned toanother frequency in the cell where the mobile station is currentlylocated and the radio frequencies in neighboring cells (in the order ofoccupancies).

Upon completion of the generation and storage of a radio frequency list,the CPU 13 a performs known processing to turn off the power (stepST33), and terminates control at power-off.

When the user designates power-on in the mobile station MS, the CPU 13 aof the mobile station MS performs start-up control as shown in FIG. 10.

Upon reception of the power-on instruction, the CPU 13 a initializes avariable X to “1” (step ST42).

The CPU 13 a then searches the radio frequency list stored in the memorysection 33 for a radio frequency whose priority level is “X” (stepST52). The CPU 13 a checks the searched-out radio frequency (step ST53),and determines whether the radio frequency can be used (step ST54). Ifthe radio frequency cannot be used, the CPU 13 a updates the variable Xto “X+1” (step ST55), and repeats the processing in steps ST52 to ST54.

If a radio frequency that can be used is found, the CPU 13 a capturesthe pilot channel of the radio frequency (step ST56). This pilot channelcapturing operation is performed by acquiring the phase and PN timingthrough the search receiver 23. Upon completion of pilot channelcapturing, the CPU 13 a receives a sync channel to receive informationindicating the system configuration and the system timing, and alsocaptures a paging channel (step ST57).

Upon completion of the above processing, the CPU 13 a enters the standbystate.

With this operation, at the time of start-up operation by power-on, theCPU 13 a enters the standby state while a radio frequency with a higherpriority level, which is set at the time of power-off in accordance withthe use state of each radio frequency, is set as a candidate frequency.If, therefore, the use state of each radio frequency at the time ofpower-on has not greatly changed from that at the time of power-off, theCPU 13 a is set in the standby state with a radio frequency with a lowoccupancy. This raises the possibility that a radio frequency with alower channel occupancy will be used for communication from now on.Eventually, therefore, the channel occupancies of the respective radiofrequencies are averaged.

Even if the use state of each radio frequency at the time of power-onhas changed from that at the time of power-off, since many mobilestations MS power off at various timings, radio frequencies are randomlyselected by the respective mobile stations MS after power-off. As aresult, the channel occupancies of the respective radio frequencies willbe averaged.

When the channel occupancies of the respective radio frequencies areaveraged in this manner, available traffic channels can be ensured inthe respective radio frequencies in many cases, thereby reliablyhandling new calls and mobile handoffs.

Mobile handoff control will be described next. A mobile station MSjlocated in a cell Z4 of the base station BS4 will be exemplified.

The CPU 13 a in the mobile station MSj performs mobile handoff controlas shown in FIG. 11 at a predetermined timing, e.g., at predeterminedperiods, during radio communication.

At the predetermined timing, the CPU 13 a detects the reception powerlevel of the pilot channel that corresponds to the currently used radiofrequency and is transmitted from the base station BS other than thebase station BS4 forming the cell Z4 in which the mobile station ispresent (step ST61). The CPU 13 a compares the detected value with apredetermined threshold (step ST62). If the reception power level of thepilot channel from the base station BS other than the base station BS4is equal to or lower than the threshold, the mobile station MSj is notlocated at the boundary between the cell Z4 and another cell. In thiscase, since no mobile handoff is required, the CPU 13 a terminates thismobile handoff control.

Assume that the mobile station MSj moves from a position I in the cellZ4 of the base station BS4 to a boundary position II between the cell Z4of the base station BS4 and a cell Z3 of the base station BS4, as shownin FIG. 12. In this case, the reception level of the pilot channel fromthe base station BS3 increases in the mobile station MSj. When thereception power of the pilot channel from the base station BS other thanthe base station BS4 exceeds the threshold, the CPU 13 a transmits amessage indicating the phase of a PN code sent over the pilot channeland the reception power level of the pilot channel to the base stationBS4 (to be referred to as a source base station hereinafter) forming thecell Z4 in which the mobile station has been located (step ST63).Thereafter, the CPU 13 a checks whether a handoff instruction is sentfrom the source base station BS4 within a predetermined period of time(step ST64).

In the base station BS4, the CPU 42 a performs mobile handoff control asshown in FIG. 11 at a predetermined timing, e.g., predetermined periods.

At the predetermined timing, the CPU 42 a checks whether the messagetransmitted from the base station MS is received (step ST71). If themessage transmitted from the mobile station MSj is received as describedabove, the CPU 42 a analyzes the message to check whether handoff isrequired (step ST72).

If it is determined that handoff is not required, or it is determined instep ST71 that no message is received, the CPU 42 a terminates mobilehandoff control. If, however, a message is received from the mobilestation MSj that has moved to a boundary position II between the cell Z4of the base station BS4 and the cell Z3 of the base station BS3 as shownin FIG. 12, handoff is required. In such as case, therefore, the CPU 42a determines the cell Z3 as a cell the mobile station MSj is enteringfrom the message, and transfers a handoff request to the base station(to be referred to as a destination base station) forming the cell Z3through the control station CS (step ST73). The CPU 42 a of thedestination base station BS4 checks the channel occupancy measured bythe channel occupancy monitoring means 42 a in the new control stationBS3 (step ST74). The CPU 42 a of the source base station BS4 then checkswhether the channel occupancy of the radio frequency currently used bythe mobile station MSj in the destination base station BS3 exceeds apredetermined threshold (step ST75).

If the channel occupancy of the radio frequency currently used by themobile station MSj in the destination base station BS3 is equal to orlower than the predetermined threshold, the radio frequency currentlyused by the mobile station MSj can also be used at the new. The CPU 42 aof the source base station BS4 therefore determines soft handoff as ahandoff method to be used, and transmits a handoff instruction forcontrolling soft handoff to the mobile station MSj (step ST76).Thereafter, the CPU 42 a of the source base station BS4 performs softhandoff by a known procedure in cooperation with the mobile station MSjand the destination base station BS3 (step ST77).

If the channel occupancy of the radio frequency currently used by themobile station MSj in the destination base station BS3 exceeds thepredetermined threshold, continuous use of the radio frequency currentlyused by the mobile station MSj in the cell it is entering is notpreferable. The CPU 42 a of the source base station BS4 determines hardhandoff as a handoff method to be used, and transmits a handoffinstruction for controlling hard handoff to the mobile station MSj (stepST78). The CPU 42 a of the source base station BS4 determines a radiofrequency whose channel occupancy in the destination base station BS3does not exceed the predetermined threshold and is lowest as a radiofrequency to be used, and notifies the mobile station MSj of this radiofrequency (step ST79). The CPU 42 a of the source base station BS4performs hard handoff by a known procedure in cooperation with themobile station MSj and the destination base station BS3 (step ST80).

If a handoff instruction transmitted from the source base station BS4 inthe above manner is received, the CPU 13 a of the mobile station MSjdetermines in step ST64 that handoff is instructed. In this case, theCPU 13 a checks whether this handoff instruction designates hard handoff(step ST65).

If the handoff instruction does not designate hard handoff, the CPU 13 aperforms soft handoff by a known procedure in cooperation with thesource base station BS4 and the destination base station BS3 (stepST66).

If the handoff instruction designates hard handoff, the CPU 13 areceives a radio frequency notification following the handoffinstruction (step ST67). The CPU 13 a then performs hard handoff to astate in which the radio frequency designated by the radio frequencynotification is used, by a known procedure, in cooperation with thesource base station BS4 and the destination base station BS3 (stepST68).

With this operation, when soft handoff cannot be performed because thereis no available traffic channel in the new cell with respect to theradio frequency currently used by the mobile station MSj, another radiofrequency that can be used is selected, and hard handoff is performed.In this case, although a slight deterioration in speech communicationquality cannot be avoided, at lest the worst case, e.g., a handofffailure, can be prevented. In addition, when one of the two radiofrequencies allocated to a given base station BS exhibits dense traffic,the radio frequency used by the mobile station that moves while usingthe radio frequency with dense traffic is switched to another radiofrequency. Therefore, the channel occupancies of the two radiofrequencies can be averaged.

As described above, according to this embodiment, measures are taken tomake the use of radio frequencies random with respect to a radiofrequency as a candidate to be used by each mobile station MS in thestandby state, a radio frequency as a candidate to be used by eachmobile station MS at power-on, and a radio frequency to be used in a newcell when each mobile station MS under radio communication moves betweencells within the same base station group. With this, the channeloccupancies of the respective radio frequencies in each base station BScan be averaged. By averaging the channel occupancies of the respectiveradio frequencies in each base station BS, traffic channels can beefficiently used.

The present invention is not limited to the above embodiment. Forexample, the above embodiment uses an arrangement like the one shown inFIG. 13(a), in which one base station BS handles both radio frequencies(e.g., f1 and f2). However, as shown in FIG. 13(b), one base station BSmay be made up of a base station apparatus 61 for handling only one(e.g., f1) of the two radio frequencies, a base station apparatus 62 forhandling only the other (e.g., f2) of the two radio frequencies, and abase station control apparatus 63 for controlling the two base stationapparatuses 61 and 62.

In each embodiment described above, the base stations forms cells havingthe same diameter. In this case, however, the same diameter includesdifferences within a predetermined range. The maximum difference is set,for example, such that the diameter of a cell having the maximumdiameter is less than twice that of a cell having the minimum diameter.

In addition, the conditions for determining the necessity of idlehandoff are not limited to those in the above embodiment. For example,an arbitrary threshold may be set.

The conditions for setting the number of mobile stations to which anidle handoff instruction is to be output are not limited those in theabove embodiment, and may be arbitrary set. In addition, the number ofmobile stations to which an idle handoff instruction is to be outputneed not always be variable, and may be constant.

Furthermore, the number of mobile stations to which an idle handoffinstruction is to be output may be set in accordance with the channeloccupancy of a carrier. In this case, it is preferable that a thresholdfor determining a channel occupancy be arbitrarily changed by manualoperation by a person in charge of maintenance and management in a basestation.

The channel occupancy monitoring means 42 a, the idle handoffcontrolling means 42 b, the frequency use state notifying means 42 c,the handoff method determining means 42 d, and the network-side handoffcontrol means 42 e, which are mounted in the base station BS in theabove embodiment, may be mounted in the control station CS.

Furthermore, the circuit arrangements of each mobile station and eachbase station, the control procedures for mobile handoff and idle hardhandoff, the contents of the control, and the like can be variouslymodified within the spirit and scope of the invention.

What is claimed is:
 1. A base station apparatus belonging to any one ofa plurality of base station groups, forming a cell having apredetermined diameter, and connected to a mobile station over CDMAradio channels of a plurality of radio frequencies allocated to saidbase station group, characterized by comprising: channel occupancymonitoring means for monitoring a channel occupancy of each of aplurality of radio frequencies allocated to said self-station; andhandoff method determining means for, when said mobile station that hasbeen located in the cell of said self-station moves to a new cell formedby another base station belonging to the same base station group towhich said self-station belongs, checking, on the basis of the channeloccupancy determined with respect to said base station forming the newcell, whether the channel occupancy of the radio frequency used by saidmobile station in the new cell is not less than a predetermined value,and determining soft handoff if the channel occupancy of the radiofrequency used by said mobile station in the new cell is less than thepredetermined value, and hard handoff if the channel occupancy of theradio frequency used by said mobile station in the new cell is not lessthan the predetermined value; and network-side handoff control means forperforming predetermined handoff control associated with said mobilestation by the method determined by said handoff method determiningmeans, and notifying said mobile station of a radio frequency whosechannel occupancy in the new cell is not more than a predeterminedvalue, when hard handoff is to be performed.